1. Field of the Invention
The present invention concerns a method of assembling a ceramic support separator element module, the elements being disposed parallel to each other with their respective ends fixed into two plates through the intermediary of flexible polymer seals.
By separator elements there is meant in this instance either porous ceramic elements making it possible to effect separation by a screening effect through pores of predetermined size, that is to say filtering, or porous ceramic elements carrying on their surface a semi-permeable membrane made from a polymer or other material making it possible to effect separation by processes such as reverse osmosis, pervaporation or any other membrane process of this type.
To be more precise, use is made of separator elements having a tubular or quasi-tubular geometry, in particular a multi-channel type geometry, as described in the following publications:
J. GILLOT and D. GARCERA--"Nouveaux Media Filtrants Ceramiques pour Microfiltration Tangentielle et Ultrafiltration"--FILTRA 84 conference, Ste Francaise de filtration--October 1984--PARIS--France. PA0 J. GILLOT--G. BRINKMAN--D. GARCERA--"New Ceramic Filter Media for Crossflow Microfiltration and Ultrafiltration"--Fourth World Filtration Congress--Apr. 22-24, 1984--OSTEND--Belgium. PA0 two separator elements disposed in parallel, PA0 two end plates to which the separator elements are fixed by means of flexible seals, and PA0 a cylindrical outer casing to which the end plates are fixed. PA0 mechanical retention of the separator elements, and PA0 sealing between the cavities for the liquid to be filtered and the permeate, in addition to various ancillary functions: PA0 absorbing by virtue of their elasticity differences between thermal expansion of the separator elements and the casing of the module, and PA0 contributing to damping of vibration of the separator elements. PA0 the axis of the element is not exactly a straight line (being curved or crooked), PA0 the cross-section of the element is not a perfect circle, but rather an approximately elliptical curve, and PA0 the dimensions and orientation of the main axes of this ellipse are not constant along the tube and vary from one tube to another. PA0 the occurrence of leaks if the transverse dimensions of the ceramic element are too small or too irregular, PA0 the impossibility of fitting elements whose transverse dimensions are too large, and PA0 the generation of very high stresses, which can be high enough to rupture the ceramic element when, after fitting, the seal is to be compressed by clamping to ensure a tight seal. PA0 seal blanks are molded from an elastomer material that is resistant to organic solvents and to corrosive liquids and has a Shore A hardness between 30 and 95 and an elastic elongation per unit length in excess of 10%, each blank having an outside surface of any form and an internal cavity with a surface matched to the ideal external shape of a separator element, except that its dimensions are slightly smaller than the corresponding dimensions of said separator element, PA0 said blanks are fitted onto the ends of said separator elements, PA0 the outside surfaces of said blanks are machined to match them to the shape of said openings in said end plates, so producing said seals, and PA0 said elements are fitted into said casing between said plates in such a way as to hold said seals in compression between said elements and said plates. PA0 seal blanks are molded from an elastomer material that is resistant to organic solvents and to corrosive liquids and has a Shore A hardness between 30 and 95 and an elastic elongation per unit length in excess of 10%, each blank having an outside surface of any form and an internal cavity with a surface matched to the ideal external shape of a separator element, except that its dimensions are slightly smaller than the corresponding dimensions of said separator element, PA0 said blanks are fitted onto the ends of said separator elements, PA0 the outside surfaces of said blanks are machined to match them to the shape of said openings in said end plates, so producing said seals, and PA0 said elements are fitted into said casing between said plates in such a way as to hold said seals in compression between said elements and said plates.
Broadly speaking, a filter module functioning according to the tangential filtration mode comprises:
The distance between the end plates (the length of the separator elements) is generally much greater than the outside diameter of the plates.
There is thus obtained an assembly which delimits a cavity in which circulates the liquid to be filtered or retentate (the external side of the end plates and the internal side of the tubes or channels of multi-channel elements) and a cavity in which the permeate is recovered (contained between the two end plates, the outside of the separator elements and the inside of the casing), the two cavities communicating only via the membrane.
The seals whereby the separator elements are assembled to the end plates thus have two main functions:
A major problem arising in implementing an assembly of this kind results from the fact that it is very difficult--or even impossible--to machine a porous ceramic without causing cracks which strongly compromise its mechanical properties, while in cases where machining is possible it is always an extremely costly process. In this respect ceramics differ markedly from other materials, such as metals, with which these problems do not arise. As they have not been machined, ceramic filter elements are not of absolutely regular shape, because of deformation or shrinkage occurring during their shaping by extrusion or pressing, their drying and their sintering. To give an example, the actual shape of a theoretically cylindrical tubular element generally differs from the ideal cylinder in a number of respects:
Also, the end of the separator element may be coated with a material such as an enamel whose function is to prevent passage of the liquid from the retentate cavity to the permeate cavity via the relatively coarse pores of the support. This enamel covers the end surface of the separator element and a portion of its lateral surface, producing a more or less regular locally increased thickness.
These imperfections of shape, although small and sometimes imperceptible to the unaided eye, nevertheless make it impossible in practise to assemble ceramic elements into a rigid end plate comprising housings of accurately dimensioned size by means of preformed polymer seals of precise shape and size. The use of such seals would have the following disadvantages:
One known method for overcoming this problem is to carry out the assembly process using a seal made from a silicone rubber which is first cast in situ in liquid form, so as to adapt to the irregular shape of the ceramic elements, and then polymerized to confer on it its final shape, being then compressed by clamping it between two metal plates.
This type of assembly is perfectly satisfactory with regard to the sealing function, the retention in place of the ceramic elements and the absorption of differential thermal expansion between the ceramic and the casing. It is of limited utility, however, in that the elastomer compounds that have to be shaped in this way by casting without pressure are unable to meet all requirements with regard to resistance to corrosive liquids and solvents.
To be of general use, this type of filter has to be usable to filter or separate corrosive fluids (acids, bases, oxidizing fluids, etc) and organic solvents (hydrocarbons and others) as well as enabling use of such fluids for cleaning the separator membrane.
The limitations of seals cast in situ are apparent in the case of silicone rubbers which are highly resistant to acids, bases and oxidizing fluids, but which when exposed to organic solvents swell considerably with deterioration of their mechanical characteristics rendering them unusable as seal materials.
An object of the present invention is to avoid these disadvantages and to provide an assembly method which makes it possible to use a wide variety of compounds for the materials of the seals.